Micro-electrical-mechanical systems (MEMS) devices come in a variety of different types and are utilized across a broad range of applications. One type of MEMS device that may be used in applications such as radio frequency (RF) circuitry is a MEMS vibrating device. A MEMS vibrating device generally includes a vibrating body supported by at least one anchor and including a piezoelectric thin-film layer in contact with one or more conductive layers. As an electrical signal is presented to one or more of the conductive layers, the piezoelectric properties of the thin-film layer cause the layer to mechanically deform. The mechanical deformation of the thin-film layer in turn causes changes in the electrical characteristics of the thin-film layer, which may be utilized by circuitry connected to the device to perform one or more functions.
FIG. 1 shows a three-dimensional view of a conventional MEMS vibrating device 10. The conventional MEMS vibrating device 10 includes a substrate 12, a number of anchors 14 formed on a top surface 16 of the substrate 12, and a vibrating body 18 suspended over the substrate 12 by one or more mechanical support members 20 attached to the anchors 14. The vibrating body 18 includes a piezoelectric thin-film layer 22, a first conductive layer 24 on a first surface of the piezoelectric thin-film layer 22 opposite the top surface 16 of the substrate 12 and a second conductive layer 26 on a second surface of the piezoelectric thin-film layer 22 opposite the first surface. The piezoelectric thin-film layer 22, the first conductive layer 24, and the second conductive layer 26 may each form part of the mechanical support members 20 and extend over the anchors 14 to provide support for the vibrating body 18. A portion of the piezoelectric thin-film layer 22 and the first conductive layer 24 may be etched away in order to provide a connection point to the second conductive layer 26 as shown in FIG. 1.
In operation, the conventional MEMS vibrating device 10 can be operated as a piezoelectric transducer or a piezoelectric and electrostatic transducer. When the conventional MEMS vibrating device 10 is operated as a piezoelectric transducer, an alternating current (AC) voltage provided to the first conductive layer 24, the second conductive layer 26, or both, causes mechanical deformations in the piezoelectric thin-film layer 22, which present an electrical impedance that is dependent on the mechanical deformations in the piezoelectric transducer between the first conductive layer 24 and the second conductive layer 26. When the conventional MEMS vibrating device 10 is operated as a piezoelectric and electrostatic transducer, an AC voltage provided to the first conductive layer 24, the second conductive layer 26, or both, causes mechanical deformations in the piezoelectric thin-film layer 22, which present an electrical impedance that is dependent on the mechanical deformations in the piezoelectric transducer between the first conductive layer 24 and the second conductive layer 26 as discussed above. Further, a direct current (DC) voltage provided to the first conductive layer 24, the second conductive layer 26, or both, causes changes in the charge of the piezoelectric thin-film layer 22, which, along with the mechanical deformations caused by the AC voltage discussed above, vary a capacitance and acoustic length between the first conductive layer 24 and the second conductive layer 26. In some cases, the DC voltage may be varied to fine tune the response of the conventional MEMS vibrating device 10 to the AC voltage. That is, the electrostatic characteristics of the conventional MEMS vibrating device 10 may be utilized to adjust or tune the piezoelectric characteristics of the conventional MEMS vibrating device 10 in some circumstances. Further, the DC voltage may be modulated with a low frequency signal that is effectively mixed with the AC voltage.
As discussed in U.S. Pat. No. 8,035,280 issued to RF Micro Devices of Greensboro, N.C., the content of which is hereby incorporated by reference in its entirety, the piezoelectric thin-film layer 22 may be periodically poled. Accordingly, FIG. 2 shows a cross-section of the vibrating body 18, which is perpendicular to a front surface 28 (shown in FIG. 1) of the vibrating body 18. As shown in FIG. 2, the piezoelectric thin-film layer 22 includes a number of adjacent domains 30. Each one of the domains 30 represents a region of the piezoelectric thin-film layer 22 in which the dipoles of the piezoelectric material are substantially oriented in the same direction. The orientation of the dipoles in each particular domain 30 may be established by a poling process, in which an electric field with a particular poling orientation is provided to the particular region, thereby aligning the dipoles therein. In FIG. 2, a first set of the domains 30 have a nominal domain orientation, while a second set of the domains 30′ have an inverted domain orientation. In general, the inverted domain is translated about 180° from the nominal domain. The domains 30 are alternated such that each nominal domain 30 is adjacent to an inverted domain 30′ and each inverted domain 30′ is in turn adjacent to a nominal domain 30.
Each domain 30 is defined by a width WD and a thickness TD. In general, the widths of the nominal domain 30 and the inverted domain 30′ do not have to be equal and may be denoted separately by WD and WD′. In a predominately lateral vibrational mode of the conventional MEMS vibrating device 10, the width WD (and/or length LD, which is shown below in FIG. 3) of the domains 30 is modulated based on a signal provided to the first conductive layer 24, the second conductive layer 26, or both. In a predominately thickness vibrational mode of the conventional MEMS vibrating device 10, the thickness TD of the domains 30 is modulated based on a signal provided to the first conductive layer 24, the second conductive layer 26, or both. In some situations, the conventional MEMS vibrating device 10 may operate in a mixed lateral and thickness vibrational mode in which both the width WD and the thickness TD of the domains 30 are modulated based on a signal provided to the first conductive layer 24, the second conductive layer 26, or both. The particular type of vibrations experienced by the conventional MEMS vibrating device 10 may be dependent on the type of materials used for the piezoelectric thin-film layer 22 as well as certain properties, such as the crystalline orientation, thereof. In general, the nominal domains 30 will move in an opposite manner to the inverted domains 30′. For example, the nominal domains 30 may expand while the inverted domains 30′ shrink, or vice versa.
Due to the orientation of the domains 30 in the periodically poled piezoelectric thin-film layer 22, the periodically poled piezoelectric thin-film layer 22 will generally experience a greater amount of mechanical deformation in response to an electrical signal than conventional piezoelectric thin-film layers having a uniform dipole orientation throughout. Accordingly, using a periodically poled piezoelectric thin-film layer 22 in the conventional MEMS vibrating device 10 allows for the use of a flat or solid first conductive layer 24 and second conductive layer 26 since a desired amount of mechanical deformation of the piezoelectric thin-film layer 22 can be achieved without specialized conductive layer configurations such as inter-digitally transduced (IDT) conductive layers. Using flat or solid conductive layers increases the power handling capability of the conventional MEMS vibrating device 10. Further, using a periodically poled piezoelectric thin-film layer 22 in the conventional MEMS vibrating device 10 may increase the frequency of operation of the conventional MEMS vibrating device 10 two-fold when compared to MEMS devices utilizing piezoelectric thin-film layers having a uniform dipole orientation throughout.
FIG. 3 shows a top view of the periodically poled piezoelectric thin-film layer 22 wherein the second conductive layer 26 has been removed in order to show the details thereof. As shown in FIG. 3, each one of the domains 30 extends between lateral surfaces 32 of the vibrating body 18 such that the domains 30 form a number of adjacent elongated rectangles. The vibrating body 18 may be defined by a length LVB width WVB. A length LD of each one of the domains 30 may be approximately equal to the length LVB of the vibrating body. Further, each one of the domains 30 may be defined by a width WD and a thickness TD, as discussed above.
FIG. 4 shows a top view of the periodically poled piezoelectric thin-film layer 22 wherein the domains 30 are substantially square in shape and oriented in a checker-board pattern. In such a case, the length LD of each one of the domains 30 is about equal to the width WD of the domains 30. Using the pattern shown in FIG. 4 for the domains 30 may further increase the amount of mechanical deformation of the piezoelectric thin-film layer 22 in response to an electrical signal applied thereto, which may be desirable in certain applications.
While using periodically poled piezoelectric thin-film layers in MEMS devices may lead to performance enhancements thereof, the particular functionality of these MEMS devices is still rather limited. For example, the conventional MEMS vibrating device 10 discussed above has only limited functionality as a piezoelectric transducer and/or an electrostatic transducer. Accordingly, there is a need for MEMS devices that are adaptable to a wide variety of applications including electrical signal processing, mechanical signal processing, optical signal processing, electro-magnetic signal processing, wireless signal processing, and the like.